Reform UK’s Scotland deputy leader Thomas Kerr said: “Only weeks ago, Anas Sarwar stood before the people of Glasgow asking for their trust and insisting that his future was here in Scotland. Today, those same voters will rightly feel betrayed.
“Throughout the election campaign, he dismissed suggestions that he had ambitions elsewhere and claimed his focus was on serving Scotland.
“Now, at the first opportunity, he has walked away from the very people who placed their faith in him.”
And Scottish Green Party co-leader Gillian Mackay accused Sarwar of being a “chancer” who had “gone from promising to scrap the House of Lords to taking a seat in it”.
She added: “By grabbing a place on the unelected red benches, he’s ensured that he gets all of the power and privilege without having to worry about what voters might think.
“It is clearly not right or fair for politicians who have been rejected by the country to end up as law makers for the rest of their lives safe in the knowledge that there is nothing anyone can do to stop them.”
Jim Murphy, who was Scottish Labour leader from 2014 to 2015, told BBC News: “If it’s confirmed I think it’s good news for him [Sarwar] personally but it probably, on balance, strengthens the government.
“It will open up a vacancy, of course, for leader of the Scottish Labour Party.”
He said Andy Burnham had a maximum three years until the next election and had appointed people he felt could deliver quickly.
Murphy added: “He’s come to a judgment that Anas, as part of a wider team, can do that.
“That will be difficult for him [Sarwar]. He will have to get up to speed very quickly but congratulations to him.”
